Here is an easy to strategy to beating Emerald

Pre-Fight Requisites
1) Master 6 MIME and Counter Materia (The counter command blue one)
2) Give pair MIME and Counter 6 times on Cloud
3) Give Cloud 2 HP Plus Materia (both mastered so ur HP will be 9999.
4) Make sure Cloud is wearing his Ultimate Weapon, and has Limit Break 4
5) Before fight get Clouds Omnislash ready

Emerald Weapon Battle
6) Start by giving Cloud a Hero Drink, this will make him hit harder to get 9999 per hit on Omnislash

7) Use Omnislash, he will hit for 10k per slash 15 times
10x 15 = 150k

8) Because you have 6 Mime-Counter combos Cloud will continue to use Omnislash 6 times

6 x 150x = 900k

9) The total damage done will be over 1 million and kill Emerald instantly.

Here's a video showing this strategy...
Final Fantasy VII Playthrough Part 67 Emerald Weapon - YouTube

Using this strat you can beat Emerald in under 5 minutes and don't even need to be level 99